In an electric machine having a high terminal power, such as e.g. a power plant generator, electrical conductors having a high electrical conductivity are conventionally installed. If the cross section of an electrical conductor is increased, the conductivity does not increase to the same extent, however, on account of eddy currents occurring in the conductor. Therefore, a so-called Roebel bar is used in the electric machine, said Roebel bar having a transposition of electrical subconductors, each of the subconductors being sheathed by electrical subconductor insulation. In this case, the winding can additionally be sheathed by main insulation.
During the operation of the machine, the Roebel bar heats up, in particular owing to the current flowing through it. In order to prolong the service life of the Roebel bar, the Roebel bar is designed such that high temperatures are avoided. For this purpose, knowledge of the temperature of the Roebel bar during the operation of the machine is required; in particular, knowledge of the temperature of the hottest location, the so-called hot spot is required. The hot spot is situated between the hottest subconductor and its subconductor insulation.
Various methods are available for measuring the temperature of the Roebel bar. Conventionally, an electrical resistance thermometer or a thermoelement is introduced into the separator of the winding. However, the resistance thermometer or the thermoelement is not suitable for measuring the temperature directly at the subconductor, rather they are only used to measure temperatures within the slot but outside the main insulation.
As an alternative, an optical fiber is used, which is not electrically conductive. In this case an optical fiber is used with a so-called Bragg sensor arranged at its end. In the Bragg sensor, a periodic modulation of the refractive index is written into the optical fiber by means of a laser. This periodic modulation acts like an interference filter in which, after the interference filter has been exposed to light, light having a predetermined spectrum is reflected back. The central wavelength of the spectrum depends on the distance between two adjacent refractive index maxima in the periodic modulation. In the event of an increase in temperature, the Bragg sensor expands, as a result of which the distance between the refractive index maxima increases and the central wavelength of the spectrum changes.
A light source is arranged at that end of the optical fiber which is remote from the Bragg sensor, said light source being used to expose the Bragg sensor to light. The spectrum of the light reflected back from the Bragg sensor is measured and the temperature of the Bragg sensor is determined from the spectrum. If the Bragg sensor is fitted to a subconductor composed of copper, then a differential expansion arises between the sensor and the subconductor on account of the increase in temperature, since glass and copper have different coefficients of thermal expansion. The differential stress disturbs the measurement, with the result that the accuracy of the measurement decreases greatly. A remedy is provided by the Bragg sensor being sheathed by a glass capillary in which the Bragg sensor can expand without experiencing external stresses.
The Bragg sensor together with the glass capillary has such a large extent that it cannot be introduced between the subconductor and the subconductor insulation, but rather is introduced the winding between the subconductor insulations. As a result, the distance between the Bragg sensor and the hot spot is approximately 2 mm to 3 mm. The accuracy of the measurement of the temperature of the hot spot decreases greatly as consequence.